Many of the targets for pharmaceutical drug discovery are ligands for receptor proteins, many of which have recently been cloned and pharmacologically characterized. Now that a large number of receptors have been cloned, a major goal of the pharmaceutical industry is to identify ligands for these receptors by screening vast libraries of substances. Unfortunately, with available methods and technology, a major limitation in the drug discovery process is the time and expense required to screen these libraries against so many targets.
The first step in the characterization of ligand interaction with a cloned receptor is to express the receptor in a ligand sensitive form. While a few receptors can be expressed in easily manipulated model systems such as yeast and E. coli, the interactions of ligands with most receptors are influenced by postranslational modifications that are only present in mammalian cells, and many of these receptors require mammalian proteins to accurately transduce their biological effects. Thus for wide applicability, an assay system must be based on expression of cloned receptors in mammalian cells. The ability of ligands to interact with receptors can be evaluated by competition with a labeled ligand (eg. radionucleotide) for a binding site on the receptor. Such assays are popular because they involve relatively few steps. Also, since binding often does not require interaction with other cellular proteins, these assays are less sensitive to factors such as levels of expression of the receptor and the cellular environment. Recently, technology such as the Proximity Assay (Amersham Co) has further simplified these assays making automation and mass screening possible. Binding assays have many limitations: (i) For many technical reasons, binding assays are almost always performed in nonphysiological buffers. These buffers often markedly influence receptor pharmacology. (ii) Agonists and antagonists are not reliably discriminated in binding assays. (iii) Only binding sites for which labeled ligands are available can be studied. (iv) Since only modest levels of receptor (binding site) expression have been achieved in mammalian cells, propagation of receptors is a major expense in these assays. (v) The vast majority of labeled ligands are radioisotopes, the purchase, handling and disposal of which are major expenses.
To reliably discriminate between agonist and antagonist ligands, a response of the receptor must be measured. Responses to agonist activation of receptors are commonly measured as altered activity of various endogenous cellular proteins. Examples include measurement of second messengers such as cAMP (adenylyl cyclase), phosphoinositol metabolism (phospholipase c), tyrosine phosphorylation, and ion channels. All of these assays require the use of cells and/or cellular preparations that have a high degree of biological integrity, and these assays include many complex and expensive steps (Schlessinger and Ullrich, Neuron 9, 383 (1992); chapters in Molecular Biology of G-protein-coupled receptors, M Brann, ed., Birkhauser (1992)).
A strategy that has been used to avoid the time and expense of measurement of endogenous proteins is to express conveniently assayed marker proteins that can be controlled by activation of the receptor. For example, receptors that control levels of transcription factors can be assayed using markers whose expression is under the transcriptional control of these factors. While this approach has led to convenient assays of receptors that are known to function as controllers of transcription (eg. steroid/thyroid hormone receptors, Evans (WO 91/07488); Spanjaard et al. Mol. Endocrinology 7:12-16 (1993)), these assays have proven to have limited utility when applied to cell surface receptors, presumably because of the more modest transcriptional control that these receptors exert. Other than the assays that are based on transcriptional control, no approach has been described to assay receptors via recombinant markers that can be conveniently measured.
Another approach is to express the receptors in specialized cells that have endogenous response mechanisms that allow convenient assay of ligand activation of the receptor. Two examples include the RBL cells and melanophores. In RBL cells, muscarinic receptors that stimulate phospholipase c enhance the release of the enzyme hexosaminidase (Jones et al., FEBS Lett. 289, 47 (1991)), a conveniently measured response. In melanophores (cultured pigment cells) cloned receptors that change cAMP levels alter cellular color, a response that is similarly easily measured (Potenza et al., Anal. Biochem. 206, 315 (1992)). The limitations of these assays are that only certain functional types of receptors can be measured. Also, while the assays are relatively convenient, there are limitations inherent in the endogenous responses and cells that are used.
When exposed to ligands, a wide diversity of receptors are able to alter the pH of the media that is used for cell culture. These pH changes are small in magnitude and require expensive instrumentation for measurement (Cytosensor, Molecular Dynamics Co.). This device is not compatible with other instruments that are used in mass screening (eg. use of a 96 well plate format) and because samples must be incubated within the instrument for several minutes, there is limited sample throughput.
A theoretical limitation inherent in all of the above assays is the inability to assay a given ligand against more than than a few receptors at the same time. For example, radioligand binding assays can only be multiplexed to the extent that different and distinguishable radioisotopes are available (eg. .sup.3 H versus .sup.125 I). Because of their limited dynamic range, incompatible assay conditions, and the fact that many receptors cannot be distinguished from one another based on their functional responses, second messenger responses, and most other biochemical effects of receptors, are not at all amenable to multiplexed assay. Similarly, the RBL assay, melanophore assay, and Cytosenor pH assays, are only applicable to assay of a single receptor at a time.
Another cellular response that is shared by many receptors is the ability to alter cellular growth. NIH 3T3 cells are a fibroblast cell line that has been extensively used to evaluate the activity of large diversity of gene products that control cell growth, and a number of receptors are able to control the activity of these cells when stimulated by individual ligands. Examples include nerve growth factor (NGF) which stimulates growth only when these cells have been transfected with trk A receptors (NGF receptor) (Cordon-Cardo et al., Cell 66:173-183 (1992); Chao, Neuron 9:583-593 (1992)), carbachol (a muscarinic agonist) stimulates cells transfected with certain muscarinic receptors (Gutkind et al., Proc. Natl. Acad. Sci. USA 88, 4703 (1991); Stephens et al., Oncogene 8, 19-26 (1993)), and norepinephrine stimulates cells transfected with certain alpha adrenergic receptors (Allen et al., Proc. Natl. Acad. Sci. USA 88, 11354 (1991)). After long-term stimulation with agonist ligands, the cells change a number of characteristics including cellular growth, loss of contact inhibition, and formation of macroscopic colonies called foci. The ability to induce foci in NIH 3T3 cells is a common characteristic of cancer-associated genes (oncogenes).
The ability of receptors and other gene products to stimulate growth and induce foci in NIH 3T3 cells correlates with the stimulation of individual second messenger systems. Trk A receptors stimulate tyrosine phosphorylation (tyrosine kinase receptor), and many other genes that stimulate tyrosine phosphorylation stimulate growth and focus production in NIH 3T3 cells (Schlessinger and Ullrich, Neuron 9, 383 (1992)). Certain muscarinic (Gutkind et al., Proc. Natl. Acad. Sci. USA 88, 4703 (1991)), adrenergic (Allen et al., Proc. Natl. Acad. Sci. USA 88, 11354 (1991)) and serotonergic (Julius et al., Science 244, 1057 (1989)) receptors that stimulate phospholipase c, also stimulate growth and focus formation in NIH 3T3 cells. In the case of the muscarinic receptors, the ability to stimulate foci and phospholipase c have exactly the same dose/response characteristics, suggesting that these responses may be used as assays for ligand interactions. Unfortunately, these assays offer few advantages to the approaches described above. Focus assays involve a response that requires at least two weeks of cell culture, and are confounded by qualitative changes in patterns of growth. Direct measurement of cellular growth has also been used to measure effects of ligands. The most commonly used assay is .sup.3 H-thymidine incorporation (Stephens et al., Oncogene 8, 1993, pp. 19-26). These assays are neither convenient nor inexpensive to perform.